The present invention relates to an electron gun for color cathode ray tube, particularly to an electrode structure forming a main lens of in-line type electron gun and more specifically to an electron gun for color cathode ray tube which reduces generation of astigmatism, has good static convergence characteristic and also provides a structure easily ensuring highly accurate assembling.
An outline of structure of a color cathode ray tube will be explained with reference to the accompanying drawings.
FIG. 1 is a structural diagram of a color cathode ray tube of the prior art.
In this figure, a phosphor surface 3 formed by alternate coating of striped three-color phosphor materials is supported at the internal wall of face plate 2 of an external glass enclosure 1. The center axes 15, 16, 17 of the cathodes 6, 7, 8 respectively match the center axes of the apertures corresponding to a first grid electrode (G1) 9, a second grid electrode (G2) 10, a third grid electrode (G3) forming a main lens and the cathode of a shield cup electrode 13 and these are also arranged almost in parallel with each other on the common plane. The center axis 16 also matches with the center axis of the electron gun as a whole.
The center axis of the aperture at the center of a fourth grid electrode (G4) 12, which is the other electrode forming the main lens, matches with the center axis 16 but the center axes 18, 19 of both side apertures do not match with the corresponding center axes 15, 17 and are deviated a little outwardly.
Three electron beams emitted from respective cathodes enter the main lens along the center axes 15, 16, 17. The G3 electrode 11 is set to a voltage lower than that of G4 electrode 12, while the high voltage G4 electrode 12 is set to the voltage equal to that of the shield cup 13 and a conductive film 5 provided within the glass enclosure. Since the apertures at the center of both G3 electrode 11 and G4 electrode 12 are provided coaxially, the main lens formed at the center of both electrodes becomes symmetrically about the axis and thereby the center beam is once focused by the main lens and then runs straight on the orbit along the axis. Meanwhile, the side apertures of both electrodes are deviated axially with each other and therefore a field element is formed asymmetrically about the axis in the outside of axis. Therefore, the side beam is deflected toward the center beam by the axially asymmetrical field element and receives a concentrated force toward the center beam simultaneously with the focusing effect by the main lens. Thereby three electron beams are focused on the shadow mask 4 and are overlappingly concentrated.
The operation to concentrate the beams is called the static convergence (hereinafter referred to as STC).
Moreover, each electron beam is color-selected by the shadow mask 4 and only the element which excites the phosphor material of the color corresponding to each beam passes through the apertures of shadow mask 4 and reaches the phosphor surface. Moreover, an external magnetic deflection yoke 14 is provided to scan the phosphor surface with the electron beam.
It is generally known that spherical aberration of the main lens is a factor which gives large influence on the resolution characteristic of a color cathode ray tube. It is also known that enlargement of diameter of the electrodes forming the main lens is particularly effective to reduce the spherical aberration of the main lens.
However, in the case of an in-line type electron gun as shown in FIG. 1, the cylindrical main lenses respectively corresponding to R, G, B colors are arranged on the same plane. Therefore, the diameter of aperture must be less than 2/3 of the internal diameter of neck portion accommodating the electron guns among the glass enclosure 1. The limit value of such internal diameter is further reduced, considering thickness of electrodes and problem on manufacture of electrodes.
When the internal diameter of neck portion is enlarged in view of increasing the limit value, a deflection voltage also increases. Moreover, when the aperture diameter is increased, deviation from the center of aperture and distance between center axes of beams also increase, resulting in a problem that the convergence characteristic is deteriorated. Since the aperture diameter is generally set as large as possible considering such problems, further enlargement thereof is extremely difficult.
An example of non-cylindrical main lens is described in the Japanese Laid-open Patent No. 59-215640, wherein the aperture diameter of electron guns can substantially be enlarged more than the limit value explained above.
FIG. 2 is a diagram for explaining the structure of main lens of electron gun by the prior art. The reference numeral 11 denotes a G3 electrode; 12, a G4 electrode; 101, 102, cylindrical electrodes of each electrode; 121, 122, plate electrodes of each electrode.
In the same figure, the plate electrodes 121, 122 provided at the surfaces of G3 electrode 11 and G4 electrode 12 opposed with each other are arranged backward from the opposed surface and thereby the electric field of opposed electrodes enters deeply into the plate electrodes, realizing the same effect as the aperture diameter is enlarged. However, since the horizontal diameter of the sectional view of circumferential portion of electrode is larger than the vertical diameter, the field enters remarkably in the horizontal direction. Thereby, a lens converging force of horizontal direction becomes weaker than that of the vertical direction, generating astigmatism in the electron beam. In order to correct astigmatism, the aperture is formed in the non-circular form and the aperture diameter in the horizontal direction is set smaller than that of vertical direction. Thereby, a convergent field in the horizontal sectional view can be enhanced and the converging forces in both horizontal and vertical directions are balanced to eliminate astigmatism.
The main lens portion can be assembled as follow. Namely, as shown in FIG. 3, the G4 electrode 12, G3 electrode 11, G2 electrode 10 and G1 electrode 9 are inserted into core bar jigs 21 passing through the electrode apertures, the spacers (not illustrated) are provided between the electrodes for the positioning and multiform glass 20 which is softened by heat processing is attached and welded to the fitting portions of electrodes 9.about.12.
For easy assembling of the electron gun in such a structure as shown in FIG. 2, it is required that the side portion of the aperture of the opposed regions of the G3 electrode 11 and G4 electrode 12 is formed in such a shape that the semi-circular area or a part of semi-circular area of the center axes 15, 17 of the external side beam orbit shown in FIG. 1 is extracted. The first reason is that parts of electrodes can be manufactured more easily and accuracy can also be attained more easily in comparison with the electrodes of elliptical shape. The second reason is that the core bar jig 21 shown in FIG. 3 to be used for alignment of the apertures of electrodes in the electron gun along the center axes 15, 16, 17 can be manufactured easily with higher accuracy. Namely, the sectional view of the portion of the core bar jig 21 passing through the opposed apertures of the G3 electrode 11 and G4 electrode 12 can be formed in the semi-circular shape or the shape in which the semi-circular shape is partly cut out, and moreover can be formed coaxially with the part passing through the apertures of the G1 electrode 9, G2 electrode 10 and G3 electrode 11. Thereby, partial axial deviation and the shape such as elliptical section which are difficult to be manufactured does not exist.
For instance, the G4 electrode 12 of this structure is shown in FIG. 4. Namely, when the points corresponding to the center axes of cathodes 15, 16, 17 are assumed as O, P, Q, a short side in the horizontal direction of cylindrical electrode 102 is formed at the portions between the arcuated portions 102a of the radius R.sub.1 about the points O, Q and a long side in the vertical direction thereof is formed at the straight line portion 102b separated by V from the straight line X connecting the points O and Q. Here, V=R.sub.1 Therefore, an intersecting point D of the straight line 102b an arcuate portion 102a exists on the vertical lines 115, 117 which is perpendicular to the straight line X and passes through the points O, Q.
On the other hand, the plate electrode 122 is provided with an aperture for the center beam, except for the part of both ends in the horizontal direction in contact with the cylindrical electrode 102, and the side beam apertures in both sides are surrounded by the end portion 122a of plate electrode 122 and the cylindrical electrode 102. The end portion 122a is generally formed in the elliptical shape on the plane and crosses with the point D.
Although a figure and explanation are omitted here, the G3 electrode 11 and the G4 electrode 12 have almost the same structure.
Moreover, it is desirable that the G3 electrode 11 and G4 electrode 12 have the same aperture shape of the opposed areas from the following two reasons. The first reason is that the manufacturing process of electrode parts must be simplified and the second reason is that when a constant manufacturing error is generated during manufacture of parts, the effects applied on the electron beam work in the reverse directions on the G3 electrode 11 and G4 electrode 12 respectively and thereby such effects are cancelled with each other and influence of dimensional error can be reduced.